1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copier, and more particularly to an image forming apparatus for forming a color image by superimposing toners of different colors.
2. Description of the Related Art
In recent years, an image forming apparatus such as a printer and a copier provided with a color printing function has been widespread. In color printing, a color image is formed by superimposing toners of different colors such as yellow (Y), magenta (M), cyan (C), and black (K), for instance. In forming a color image, there is known a so-called tandem color system e.g. a color printer, in which a transfer belt unit is provided, and imaging sections for forming images of the respective colors are arranged along the longitudinal direction of a transfer belt. Normally, the transfer belt unit is provided with two or more rollers for supporting the transfer belt or driving the transfer belt. Hereinafter, the rollers constituting the transfer belt unit are called as “belt constituting rollers”. The belt constituting rollers are classified into a drive roller for driving the transfer belt, a driven roller for supporting the transfer belt, a tension roller for applying a tension to the transfer belt, and the like. In the above construction, a color image is formed on the surface of the transfer belt by superimposed and successive transferring of toner images of four different colors i.e. YMCK by photosensitive drums of the respective imaging sections in accordance with a driving rotation of the drive roller. This image forming operation is called as a primary transfer operation. After the primary transfer operation, the color image formed on the belt surface is transferred onto a recording sheet by secondary transfer rollers disposed opposed to the respective photosensitive drums with respect to the transfer belt. This transfer operation is called as a secondary transfer operation.
In the tandem color system, generally, it is difficult to properly superimpose toner images of the respective colors (hereinafter, the color superimposition is sometimes called as “color tuning”), and color displacement or color unevenness, which are hereinafter called as “color displacement” collectively, is likely to occur. Fluctuation of a running speed of the transfer belt is one of the factors of making color tuning difficult. Various parameters such as fluctuation of an outer diameter of the drive roller, fluctuation of a thickness of the transfer belt, or vibration of a gear in a driving section are some of the causes of the fluctuation of the running speed of the transfer belt.